In systems of the type described above, packets are prone to transmission errors due to destructive interference caused by the multipath propagation of the signal from the transmitter to the receiver. Currently, many such systems, including systems implementing IEEE 802.11/WiFi, use retransmission methods to ensure that the data contained in the lost packets is received successfully at the receiver. In other words, these systems attempt to take advantage of multiple transmissions of the same packet, a concept known as time diversity.
One such retransmission method is known as Automatic Repeat reQuest (ARQ), in which the receiver, upon failing to decode a packet, transmits a request to the transmitter to resend the packet. To receive the packet successfully, the receiver relies on the diminishing probability of error over multiple attempts. As another example, when using the HARQ retransmission method, the receiver transmits a request to the transmitter to resend an un-decoded packet in a manner similar to that of ARQ, but additionally combines the previously received packet with the resent packet to improve the probability of decoding the data in the packet.
Due to the frequency-selective nature of slow-varying channels, such as WiFi channels, wherein a channel may remain in a deep fade for a considerable amount of time, the probability of error in each ARQ retransmission decays at a very slow rate when using a retransmission method. This is caused by a given un-decoded packet being retransmitted using the same deep-fading channel. In the case of HARQ transmissions, multiple transmissions are still required to cover the packets affected by a deep fade.
In order to improve the efficiency of retransmissions, it is therefore desirable for the channels to change over time. However, the static nature of wireless channels in certain systems, such as WiFi systems operating in an indoor environment, precludes such systems from benefiting from retransmissions in an efficient manner.